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Focal Mechanisms of Deep Low‐Frequency Earthquakes Beneath Zao Volcano, Northeast Japan, and Relationship to the 2011 Tohoku Earthquake
Author(s) -
Oikawa Genki,
Aso Naofumi,
Nakajima Junichi
Publication year - 2019
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2019gl082577
Subject(s) - seismology , geology , volcano , stress field , amplitude , magma , focal mechanism , stress (linguistics) , induced seismicity , linguistics , physics , philosophy , quantum mechanics , finite element method , thermodynamics
Small, deep low‐frequency earthquakes (LFEs) with dominant frequencies of 2–8 Hz occur at depths of 20–40 km and are thought to be related to movement of magma or crustal fluid, but their physical source processes remain largely unknown. Therefore, we determine the focal mechanisms of LFEs beneath Zao volcano, Northeast Japan, using the S / P amplitude ratio. Most focal mechanisms are classified into five groups: three types of double couples, a compensated linear vector dipole group, and a single force group. Double couples in 2007–2012 are consistent with those expected for the regional stress field, but no such events have been observed since 2013. This transition in focal mechanisms was simultaneous with a rapid increase in LFE activity beneath Zao. Our results suggest that LFEs beneath Zao were controlled mainly by the local stress field, but the stress field changed about two years after the 2011 Tohoku earthquake.